Alarm systems, such as burglar and fire alarms, utilizing cable TV communication media are known. For example, see U.S. Pat. No. 3,761,914 to Hardy et al, U.S. Pat. No. 4,114,150 to Yamazaki et al, U.S. Pat. No. 3,803,491 to Osborn and U.S. Pat. No. 4,245,245 to Matsumoto et al.
The Osborn and Matsumoto et al patents describe respective cable TV communication systems which may be classified as "polling" type systems. In such systems, individual subscribers are polled from the headend of the cable TV system by transmitting a unique address which corresponds to a particular subscriber. Alarm messages are transmitted from the addressed subscriber unit to the headend in response to the polling signal.
However, as the number of subscribers is increased, the average response time of the alarm system is increased since it takes longer to poll the larger number of subscribers. The increase in response time is particularly pronounced when the polling system includes other data transmitted between the subscriber's unit and the headend in addition to the alarm messages. System response time can be of great importance in certain types of alarms, such as medical alert, criminal attack, or the like.
In the Hardy et al and Yamazaki et al patents, alarm messages are transmitted by the subscriber unit essentially when an alarm condition is first detected, which tends to improve the system response time as compared to polling type systems. Individual subscribers are identified by the frequency of their respective transmissions. For example, in the Yamazaki et al patent, each subscriber is assigned a unique low frequency modulating signal and a high frequency carrier signal which is unique to a group of subscribers. Hardy et al suggests using telephone type touch tone audio frequencies to uniquely identify subcribers. Additional subscribers are accommodated by using the same audio frequencies to modulate additional carrier frequencies.
However, the complexity and cost of a frequency discrimination system, as typified by the Yamazaki et al and Hardy et al systems, increases as the number of subscribers is increased, requiring increased numbers of modulators, demodulators, filters, etc.. Furthermore, the use of additional frequencies to accommodate additional subscribers tends to reduce the cable spectrum space which would otherwise be available for additional cable services.